Plasma, the fourth state of matter, consists of gaseous complexes in which all or a portion of the atoms or molecules are dissociated into free electrons, ions/cations, free radicals, and neutral particles. On earth, plasma occurs naturally in lightning bolts, flames, and similar phenomena, or may be manufactured by heating a gas to high temperatures, or by applying a strong electric field to a gas, the more common method. The latter type of plasma, often referred to as an electrical discharge plasma, can be further subclassified as a “hot” plasma, i.e., dissociated gas in thermal equilibrium at high temperatures (˜5000 K), or “cold” plasma, i.e., nonthermal plasma wherein the dissociated gas is at low temperatures but its electrons are at high temperature (i.e., in a state of high kinetic energy).
The usefulness of plasma for manufacturing and other applications is best understood by reviewing common applications for cold plasma. As an example, common cold plasma processing methods are commonly used to alter the surface properties of industrial materials without affecting the bulk properties of the treated material. The most common cold plasma surface treatments may be generally categorized as cleaning, activation, grafting, and deposition processes, each of which will now be briefly reviewed.
Plasma cleaning processes typically utilize inert or oxygen plasmas (i.e., plasmas generated from inert or oxygen-based process gases) to remove contaminants (generally organic contaminants) on a material surface subjected to vacuum. The contaminants are exposed to a plasma stream, and they undergo repetitive chain scission from the plasma until their molecular weight is sufficiently low to boil away in the vacuum.
Plasma activation is used when a material (generally a polymer or elastomer) is subjected to a plasma generally produced from an inert or non-carbon gas, and results in the incorporation of different moieties of the process gas onto the surface of the material being treated. For example, the surface of polyethylene normally consists solely of carbon and hydrogen. However, if subjected to an appropriate plasma, the surface may be activated to contain a variety of functional groups which enhance the adhesion and permanence of coatings later applied to the surface. As an example, a surface can be treated to greatly enhance its ability to bond with adhesives.
Deposition, which is exemplified by a process referred to as plasma-enhanced chemical vapor deposition (PECVD), utilizes a complex molecule as the process gas. The process gas molecules are decomposed near the surface to be treated, and recombine to form a material which precipitates onto and coats the surface.
Grafting generally utilizes an inert process gas to create free radicals on the material surface, and subsequent exposure of the radicalized surface to monomers or other molecules will graft these molecules to the surface.
The foregoing cold plasma processes have numerous practical applications, including sterilizing of medical equipment, application of industrial and commercial coatings, etching computer chips, semiconductors, and circuits, and so forth. Hot plasma might be used for generally the same types of applications as cold plasma. However, hot plasma applications are limited since most organic matter cannot be treated under the high temperatures required for hot plasmas without severe degradation. Additionally, hot plasma technology is energy and equipment intensive, making it expensive and difficult to work with. In contrast, cold plasma may be used at temperature ranges as low as room temperature (or lower), making it significantly easier to handle. However, cold plasma processes have the disadvantage that they generally need low pressure conditions to operate (generally a vacuum), and consequently need large, static (i.e., immobile) equipment with a low-pressure treatment chamber to operate. This causes significant manufacturing constraints since the need to treat items within an enclosed chamber makes it inherently difficult to process the items continuously in assembly-line fashion, as opposed to processing the items in batches.
Some of these difficulties have been overcome with further developments in dielectric barrier discharge (DBD) plasma production processes. These processes, which may take place at room temperature and non-vacuum conditions, have a gas-filled cavity insulated from an opposing pair of electrodes by one or more dielectric layers. When an alternating high voltage electrical current is applied to the electrodes, “microdischarges” occur within the gas(es) in the cavity between the electrodes and dielectric layers, thereby generating plasma. DBD apparata are sometimes used to generate ozone by ionizing oxygen passing through the cavity of the apparatus, or to break apart volatile gaseous organic compounds passing through the cavity.
However, conventional DBD plasma generation apparata are limited in several respects. One important limitation of prior DBD apparata is that they are generally adapted for plasma treatment of the gas situated within or passing through the cavity, or of solid workpieces situated within or passing through the gas-filled cavity. DBD apparata are generally not regarded as being suitable for the plasma treatment of liquid-phase materials, primarily because filamentary or “streamer-type” discharges occur in the liquids instead of microdischarges, producing significant heat and leading to unwanted effects (e.g., generation of carbon in hydrocarbon process liquids). Additionally, treatment efficiency is low because such discharges only affect liquid resting along the discharge path, as opposed to the more widespread treatment effected by microdischarges (which are dispersed about the cavity between the electrodes). As a result, plasma reactions in liquids are generally performed using specialized apparata such as that described in U.S. Pat. Nos. 5,908,539 and 5,534,232. Conventional DBD plasma generation also has the disadvantage that throughput of treated workpieces (or treated gases) is limited by the allowable size of the cavity through which they must pass, and the cavity is generally quite small owing to the need for close spacing of the electrodes (and with the cavity space being further reduced by the presence of the dielectric layers, which are generally made of ceramic material). Thus, while the advantages of DBD plasma generation are compelling, it has only gained widespread acceptance in a limited number of fields. As a result, there has long been a desire for methods and apparata which provide the benefits of DBD plasma treatment, but which enhance its versatility.